What term describes the inheritance of a trait which in the heterozygote has an intermediate expression between either allele?

INCOMPLETE DOMINANCE

Explanation:

Incomplete dominance is an exception to Mendel's pattern of inheritance as he described in his LAW of DOMINANCE, that in an heterozygous state, an allele will mask the expression of its variant pair in a gene. Incomplete dominance has shown that an allele might not be completely dominant over its pair but instead form a hybrid that possesses an intermediate phenotype that is the combination of both parental phenotypes.

An example is when Mendel crossed peas that produced red flowers (RR) will peas that produced white flowers (rr). The red flower allele (R) did not show complete dominance over the white flower allele, instead a heterozygote F1 offspring (Rr) with pink flowers was produced. The pink color was an intermediate phenotype of both white and red.